README.md

@@ -82,7 +82,7 @@ The program is added step by step (one pull request per lecture step):
 | Step | Status |
 | --- | --- |
 | 1. Scrape the Billboard Hot 100 | done (below) |
-| 2. YouTube Music authentication | planned in milestone 4 |
+| 2. YouTube Music authentication | code done; live screenshot waits for `browser.json` |
 | 3. Create the playlist | planned in milestone 5 |
 | 4. Search and add songs | planned in milestone 6 |
 
@@ -102,6 +102,20 @@ The program's output for the same date (rendered from the real output of the run
 
 ![Output of step 1 for 2026-04-18](docs/wiki/images/step1-terminal-output.png)
 
+### Step 2 - YouTube Music authentication
+
+Create `browser.json` as described in [Setup](#-setup), then check the login:
+
+```bash
+python -m musical_time_machine --check-login
+```
+
+It prints `Found N playlists in your library.`. With no or an expired `browser.json` it prints the renewal steps and exits with code 1 instead of a stack trace. `--browser-file PATH` uses another file.
+
+The screenshot below shows the run **without** a `browser.json` and the tests that use a fake YouTube Music service. A screenshot of a live login is added once the file exists (issue 22 stays open until then); the file holds personal cookies and is never committed.
+
+![Login check without browser.json and the step 2 tests](docs/wiki/images/step2-check-login.png)
+
 The screenshots and the instructions of the next steps are added with each step.
 
 ## 🧪 Tests

docs/artifact-registry.md

@@ -17,7 +17,7 @@ document of a type. `Primary File` may contain a glob (e.g.
 | MIL | Milestone / Gateway | docs/milestones/mil-*.md | 008 |
 | GOV | Governance / ARB Workflow | docs/sqa/governance.md | 002 |
 | TM | Traceability Matrix | docs/sqa/traceability-matrix.md | 002 |
-| RC | SQA Review Record | docs/sqa/reviews/rc-*.md | 014 |
+| RC | SQA Review Record | docs/sqa/reviews/rc-*.md | 015 |
 
 ## Languages
 

docs/sqa/reviews/rc-014-mil-004-code.md

@@ -0,0 +1,61 @@
+# Review of the code of MIL-004
+
+## Metadata
+| Key | Value |
+| --- | --- |
+| ID | RC-014 |
+| CrossReference | [MIL-004], [QC-PY-001] |
+
+## Version History
+| Date | Status | Author | Reviewer | Change | Commit |
+| --- | --- | --- | --- | --- | --- |
+| 2026-10-28 | Proposed | Jens Tirsvad Nielsen | AI as stand-in for S01 | Initial version | pending |
+
+---
+
+## Artifact Under Review
+
+- Instance reviewed: [MIL-004] (the code of its pull request, `src/` and `tests/`)
+- Checklist used: [QC-PY-001] (`QC-PY-001`)
+- Scope: full review
+- Language and domain: n/a
+- Language reviewer: none
+- Mode: autonomous, not independent (reviewer is `AI as stand-in for S01`)
+
+## Checklist Results
+
+| # | Criterion | Status | Evidence/Notes |
+| --- | --- | --- | --- |
+| 1 | Packages, modules, functions, variables, classes and constants follow PEP 8 casing (`snake_case`, `PascalCase`, `UPPER_SNAKE`) | Pass | PascalCase classes, snake_case functions, UPPER_SNAKE constants. |
+| 2 | Names state purpose in the domain's language; no unexplained abbreviations, no single-letter names outside tiny scopes | Pass | Names state purpose (BrowserFileMissingError, count_library_playlists). |
+| 3 | Code is produced by the project's formatter and passes its linter with no unexplained suppressions | Pass | ruff format and ruff check report no finding. |
+| 4 | Every function and method signature is type-annotated, including `-> None` | Pass | All functions annotated; the one YTMusic default factory is typed by the YTMusicLike protocol. |
+| 5 | No bare `except:`, no swallowed exceptions; specific exceptions are raised and the cause is kept (`raise ... from`) | Pass | Specific ytmusicapi errors are caught and re-raised as AuthenticationError with `from`. |
+| 6 | No mutable default arguments and no shadowed builtins | Pass | Checked in the diff. |
+| 7 | Files, locks and connections are managed with context managers | N-A | The code opens no file; it checks Path.exists and passes the path to ytmusicapi. |
+| 8 | Public modules, classes and functions have docstrings that say what, not how | Pass | Checked in the diff. |
+| 9 | Logging uses `logging`, not `print`; no secrets or personal data in log output | Pass | No print in the source except the injected writer; browser.json content is never read or logged. |
+| 10 | Classes and operations trace to the Design Class Diagram they implement; deviations are recorded | N-A | No Design Class Diagram exists; all tasks are technical. |
+| 11 | Tests exist for new behaviour, are named for the behaviour, and do not depend on order or the network | Pass | 47 tests, 97 % coverage, no network; the missing-file test uses tmp_path; browser.json is never created. |
+| 12 | Type checker runs in strict mode without errors; `Any` is justified in a comment | N-A | mypy is not configured; no Any in the source. Optional criterion. |
+| 13 | Dependencies are declared and pinned in the project's dependency file, none unused | Pass | ytmusicapi is now used by infrastructure. |
+| 14 | Each piece of knowledge (a business rule, constant, format, validation or query) is defined in one place; code is merged only where it expresses the same knowledge, not where it merely looks alike | Pass | Messages and the file name are constants defined once. |
+| 15 | Imports point inward: business-rule modules import no framework, database, UI or delivery-mechanism code, and there are no import cycles | Pass | The port is in application; only infrastructure imports ytmusicapi. |
+| 16 | SOLID holds: a class or module has one reason to change; new behaviour is added by extension, not by editing a type test in several places; subclasses honour the contract of their base; protocols are narrow; high-level code depends on protocols or abstract types and receives its implementations | Pass | MusicService is a narrow Protocol; the CLI depends on it. |
+| 17 | Source is laid out in the layers and folders of the clean architecture folder structure (`domain`, `application`, `infrastructure`, `presentation` packages under `src/`, and `unit`, `integration` and `e2e` under `tests/`), in `snake_case`, and imports between the layers point inward | Pass | Layer folders and tests/unit/<layer> exist. |
+| 18 | File and folder names are English, contain no space and no character outside letters, digits, `-`, `_` and `.`, and follow the casing of the language for files | Pass | English snake_case file names. |
+
+## Overall Verdict
+
+Go — every Mandatory criterion passes or does not apply; no blocking finding.
+
+## Action Items
+
+| Action | Owner | Due |
+| --- | --- | --- |
+| None | S01 | - |
+
+---
+
+[MIL-004]: ../../milestones/mil-004-step-2-youtube-music-auth.md
+[QC-PY-001]: ../../../framework/qc/code/qc-programming-python.md

docs/sqa/traceability-matrix.md

@@ -28,7 +28,7 @@ Tracks backward and forward links between artifact instances so that the traceab
 | [MIL-001] | MIL | en | it | [BC-001], [PP-001] | [MIL-002] | [RC-005] |
 | [MIL-002] | MIL | en | it | [MIL-001] | [MIL-003] | [RC-006], [RC-012] |
 | [MIL-003] | MIL | en | it | [MIL-002] | [MIL-004] | [RC-007], [RC-013] |
-| [MIL-004] | MIL | en | it | [MIL-003] | [MIL-005] | [RC-008] |
+| [MIL-004] | MIL | en | it | [MIL-003] | [MIL-005] | [RC-008], [RC-014] |
 | [MIL-005] | MIL | en | it | [MIL-004] | [MIL-006] | [RC-009] |
 | [MIL-006] | MIL | en | it | [MIL-005] | [MIL-007] | [RC-010] |
 | [MIL-007] | MIL | en | it | [MIL-006] | - | [RC-011] |
@@ -83,3 +83,4 @@ Tracks backward and forward links between artifact instances so that the traceab
 [01e016e]: https://git.tirsystem.com/Tirsvad-Udemy-100-days-of-code/046-scraping-the-billboard/commit/01e016e15dd93cface4f4ae9e9e99919e0aace2b
 [RC-012]: ./reviews/rc-012-mil-002-code.md
 [RC-013]: ./reviews/rc-013-mil-003-code.md
+[RC-014]: ./reviews/rc-014-mil-004-code.md

src/musical_time_machine/application/ports.py

@@ -5,6 +5,7 @@
 from datetime import date
 from typing import Protocol
 
+from musical_time_machine.domain.playlist import Playlist
 from musical_time_machine.domain.song import Song
 
 
@@ -18,3 +19,14 @@ class ChartSource(Protocol):
         @return The songs in chart order.
         """
         ...
+
+
+class MusicService(Protocol):
+    """@brief The user's music streaming account."""
+
+    def library_playlists(self) -> list[Playlist]:
+        """@brief Get every playlist in the user's library.
+
+        @return The playlists of the library.
+        """
+        ...

src/musical_time_machine/application/use_cases.py

@@ -4,7 +4,7 @@
 
 from datetime import date
 
-from musical_time_machine.application.ports import ChartSource
+from musical_time_machine.application.ports import ChartSource, MusicService
 from musical_time_machine.domain.song import Song
 
 
@@ -26,3 +26,12 @@ def scrape_titles(chart_date: date, source: ChartSource) -> list[str]:
     @return The titles in chart order.
     """
     return [song.title for song in scrape_songs(chart_date, source)]
+
+
+def count_library_playlists(service: MusicService) -> int:
+    """@brief Count the playlists in the user's library.
+
+    @param service The music service to ask.
+    @return The number of playlists.
+    """
+    return len(service.library_playlists())

src/musical_time_machine/constants.py

@@ -92,3 +92,37 @@ SEARCH_FILTER: Final[str] = "songs"
 ## @brief Privacy status of the created playlist.
 PLAYLIST_PRIVACY: Final[str] = "PRIVATE"
 ## @}
+
+## @name Login check
+## @{
+
+## @brief Message with the number of library playlists; format with the count.
+PLAYLIST_COUNT_MESSAGE: Final[str] = "Found {count} playlists in your library."
+
+## @brief Message when browser.json is missing or rejected; format with reason and file.
+BROWSER_RENEWAL_MESSAGE: Final[str] = (
+    "YouTube Music login failed: {reason}\n"
+    "To create or renew {file}:\n"
+    "  1. Open Firefox and go to https://music.youtube.com while logged in.\n"
+    "  2. Open the developer tools (F12) and select the Network tab.\n"
+    "  3. Filter the requests by 'browse' and select one POST request.\n"
+    "  4. Right-click it and choose Copy Request Headers.\n"
+    "  5. Run 'ytmusicapi browser' and paste the headers "
+    "(macOS: pbpaste | ytmusicapi browser)."
+)
+
+## @brief Reason when the browser file does not exist; format with the file.
+BROWSER_FILE_MISSING_REASON: Final[str] = "{file} was not found."
+
+## @brief Command line flag that checks the YouTube Music login.
+CHECK_LOGIN_FLAG: Final[str] = "--check-login"
+
+## @brief Help text of the check-login flag.
+CHECK_LOGIN_HELP: Final[str] = "Log in to YouTube Music and print the number of library playlists."
+
+## @brief Command line flag that names the browser file.
+BROWSER_FILE_FLAG: Final[str] = "--browser-file"
+
+## @brief Help text of the browser-file flag.
+BROWSER_FILE_HELP: Final[str] = "Path of the ytmusicapi browser headers file."
+## @}

src/musical_time_machine/domain/errors.py

@@ -17,3 +17,11 @@ class ChartUnavailableError(MusicalTimeMachineError):
 
 class ChartParseError(MusicalTimeMachineError):
     """@brief The chart page holds no songs."""
+
+
+class BrowserFileMissingError(MusicalTimeMachineError):
+    """@brief The browser authentication file does not exist."""
+
+
+class AuthenticationError(MusicalTimeMachineError):
+    """@brief YouTube Music rejected the browser authentication headers."""

src/musical_time_machine/domain/playlist.py

@@ -0,0 +1,17 @@
+"""@file
+@brief The Playlist value object.
+"""
+
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True, slots=True)
+class Playlist:
+    """@brief One playlist of the user's library.
+
+    @param playlist_id Identifier given by the music service.
+    @param title Playlist title.
+    """
+
+    playlist_id: str
+    title: str

src/musical_time_machine/infrastructure/ytmusic.py

@@ -0,0 +1,90 @@
+"""@file
+@brief YouTube Music (ytmusicapi) as a music service.
+"""
+
+import logging
+from collections.abc import Callable
+from pathlib import Path
+from typing import Any, Protocol
+
+from ytmusicapi import YTMusic
+from ytmusicapi.exceptions import YTMusicError
+
+from musical_time_machine.constants import (
+    BROWSER_FILE_MISSING_REASON,
+    BROWSER_FILE_NAME,
+    BROWSER_RENEWAL_MESSAGE,
+)
+from musical_time_machine.domain.errors import AuthenticationError, BrowserFileMissingError
+from musical_time_machine.domain.playlist import Playlist
+
+_logger = logging.getLogger(__name__)
+
+## ytmusicapi errors plus the parsing errors a stale or malformed headers file causes.
+_CLIENT_ERRORS = (YTMusicError, KeyError, ValueError)
+
+
+class YTMusicLike(Protocol):
+    """@brief The part of the ytmusicapi client this adapter uses."""
+
+    def get_library_playlists(
+        self,
+        limit: int | None = ...,
+    ) -> list[dict[str, Any]]:  # ytmusicapi returns untyped JSON dictionaries
+        """@brief List the library playlists as raw dictionaries."""
+        ...
+
+
+class YouTubeMusicService:
+    """@brief MusicService backed by ytmusicapi and a browser headers file."""
+
+    def __init__(
+        self,
+        browser_file: Path = Path(BROWSER_FILE_NAME),
+        client_factory: Callable[[str], YTMusicLike] = YTMusic,
+    ) -> None:
+        """@brief Create the service; the network is not touched until first use.
+
+        @param browser_file File with the browser request headers.
+        @param client_factory Builds a client from the path of the browser file.
+        """
+        self._browser_file = browser_file
+        self._client_factory = client_factory
+        self._client: YTMusicLike | None = None
+
+    def library_playlists(self) -> list[Playlist]:
+        """@brief Get every playlist in the user's library.
+
+        @return The playlists of the library.
+        @throws BrowserFileMissingError when the browser file does not exist.
+        @throws AuthenticationError when YouTube Music rejects the headers.
+        """
+        client = self._connect()
+        try:
+            raw = client.get_library_playlists(limit=None)
+            return [
+                Playlist(playlist_id=item["playlistId"], title=item["title"])
+                for item in raw
+                if item.get("playlistId") and item.get("title")
+            ]
+        except _CLIENT_ERRORS as error:
+            raise self._rejected(error) from error
+
+    def _connect(self) -> YTMusicLike:
+        if self._client is not None:
+            return self._client
+        if not self._browser_file.is_file():
+            reason = BROWSER_FILE_MISSING_REASON.format(file=self._browser_file)
+            raise BrowserFileMissingError(self._renewal(reason))
+        try:
+            self._client = self._client_factory(str(self._browser_file))
+        except _CLIENT_ERRORS as error:
+            raise self._rejected(error) from error
+        return self._client
+
+    def _rejected(self, error: Exception) -> AuthenticationError:
+        _logger.debug("YouTube Music rejected the login", exc_info=error)
+        return AuthenticationError(self._renewal(str(error)))
+
+    def _renewal(self, reason: str) -> str:
+        return BROWSER_RENEWAL_MESSAGE.format(reason=reason, file=self._browser_file)

src/musical_time_machine/presentation/cli.py

@@ -2,32 +2,70 @@
 @brief Command line entry point and composition root.
 """
 
+import argparse
 from collections.abc import Callable
+from pathlib import Path
 
-from musical_time_machine.application.ports import ChartSource
-from musical_time_machine.application.use_cases import scrape_songs
-from musical_time_machine.constants import DATE_PROMPT, EXIT_FAILURE, EXIT_OK, SONG_LINE_TEMPLATE
+from musical_time_machine.application.ports import ChartSource, MusicService
+from musical_time_machine.application.use_cases import count_library_playlists, scrape_songs
+from musical_time_machine.constants import (
+    BROWSER_FILE_FLAG,
+    BROWSER_FILE_HELP,
+    BROWSER_FILE_NAME,
+    CHECK_LOGIN_FLAG,
+    CHECK_LOGIN_HELP,
+    DATE_PROMPT,
+    EXIT_FAILURE,
+    EXIT_OK,
+    PLAYLIST_COUNT_MESSAGE,
+    SONG_LINE_TEMPLATE,
+)
 from musical_time_machine.domain.chart_date import parse_chart_date
 from musical_time_machine.domain.errors import MusicalTimeMachineError
 from musical_time_machine.infrastructure.bakeboard import BakeboardChartSource
+from musical_time_machine.infrastructure.ytmusic import YouTubeMusicService
+
+
+def _parse_args(argv: list[str] | None) -> argparse.Namespace:
+    parser = argparse.ArgumentParser(prog="musical_time_machine")
+    parser.add_argument(CHECK_LOGIN_FLAG, action="store_true", help=CHECK_LOGIN_HELP)
+    parser.add_argument(
+        BROWSER_FILE_FLAG, type=Path, default=Path(BROWSER_FILE_NAME), help=BROWSER_FILE_HELP
+    )
+    return parser.parse_args(argv)
+
+
+def _check_login(service: MusicService, write: Callable[[str], None]) -> int:
+    try:
+        count = count_library_playlists(service)
+    except MusicalTimeMachineError as error:
+        write(str(error))
+        return EXIT_FAILURE
+    write(PLAYLIST_COUNT_MESSAGE.format(count=count))
+    return EXIT_OK
 
 
 def main(
     argv: list[str] | None = None,
     *,
     source: ChartSource | None = None,
+    music_service: MusicService | None = None,
     read: Callable[[str], str] = input,
     write: Callable[[str], None] = print,
 ) -> int:
-    """@brief Ask for a date and print the songs of that chart.
+    """@brief Ask for a date and print the songs of that chart, or check the YouTube Music login.
 
-    @param argv Command line arguments; unused, kept for the entry point contract.
+    @param argv Command line arguments: `--check-login` and `--browser-file PATH`.
     @param source Chart source; the Bakeboard site when omitted.
+    @param music_service Music service for `--check-login`; YouTube Music when omitted.
     @param read Function that prompts and returns the user's answer.
     @param write Function that shows one line to the user.
     @return Process exit code: 0 on success, 1 when a domain error stopped the run.
     """
-    del argv
+    args = _parse_args(argv)
+    if args.check_login:
+        service = music_service or YouTubeMusicService(browser_file=args.browser_file)
+        return _check_login(service, write)
     chart_source = source if source is not None else BakeboardChartSource()
     try:
         chart_date = parse_chart_date(read(DATE_PROMPT))

tests/conftest.py

@@ -7,6 +7,7 @@ from pathlib import Path
 
 import pytest
 
+from musical_time_machine.domain.playlist import Playlist
 from musical_time_machine.domain.song import Song
 
 FIXTURE_PAGE = Path(__file__).parent / "fixtures" / "bakeboard-2026-04-18.html"
@@ -31,3 +32,18 @@ class FakeChartSource:
         if self._error is not None:
             raise self._error
         return self._songs
+
+
+class FakeMusicService:
+    """@brief MusicService returning fixed playlists, or raising a given error."""
+
+    def __init__(
+        self, playlists: list[Playlist] | None = None, error: Exception | None = None
+    ) -> None:
+        self._playlists = playlists or []
+        self._error = error
+
+    def library_playlists(self) -> list[Playlist]:
+        if self._error is not None:
+            raise self._error
+        return self._playlists

tests/unit/application/test_count_library_playlists.py

@@ -0,0 +1,25 @@
+"""@file
+@brief Tests of the count_library_playlists use case.
+"""
+
+import pytest
+from conftest import FakeMusicService
+
+from musical_time_machine.application.use_cases import count_library_playlists
+from musical_time_machine.domain.errors import AuthenticationError
+from musical_time_machine.domain.playlist import Playlist
+
+
+def test_count_library_playlists_returns_number_of_playlists() -> None:
+    service = FakeMusicService([Playlist("a", "A"), Playlist("b", "B")])
+
+    assert count_library_playlists(service) == 2
+
+
+def test_count_library_playlists_returns_zero_for_empty_library() -> None:
+    assert count_library_playlists(FakeMusicService()) == 0
+
+
+def test_count_library_playlists_propagates_authentication_error() -> None:
+    with pytest.raises(AuthenticationError):
+        count_library_playlists(FakeMusicService(error=AuthenticationError("no")))

tests/unit/infrastructure/test_ytmusic.py

@@ -0,0 +1,110 @@
+"""@file
+@brief Tests of the YouTube Music adapter with a fake client factory.
+"""
+
+from pathlib import Path
+from typing import Any
+
+import pytest
+from ytmusicapi.exceptions import YTMusicUserError
+
+from musical_time_machine.domain.errors import AuthenticationError, BrowserFileMissingError
+from musical_time_machine.domain.playlist import Playlist
+from musical_time_machine.infrastructure.ytmusic import YouTubeMusicService, YTMusicLike
+
+
+class _FakeClient:
+    def __init__(self, items: list[dict[str, Any]], error: Exception | None = None) -> None:
+        self._items = items
+        self._error = error
+        self.limits: list[int | None] = []
+
+    def get_library_playlists(self, limit: int | None = 25) -> list[dict[str, Any]]:
+        self.limits.append(limit)
+        if self._error is not None:
+            raise self._error
+        return self._items
+
+
+def _browser_file(tmp_path: Path) -> Path:
+    path = tmp_path / "browser.json"
+    path.write_text("{}", encoding="utf-8")
+    return path
+
+
+def test_constructor_does_not_call_the_factory(tmp_path: Path) -> None:
+    calls: list[str] = []
+
+    def factory(auth: str) -> YTMusicLike:
+        calls.append(auth)
+        return _FakeClient([])
+
+    YouTubeMusicService(tmp_path / "missing.json", factory)
+
+    assert calls == []
+
+
+def test_library_playlists_maps_entries_and_asks_for_all(tmp_path: Path) -> None:
+    client = _FakeClient(
+        [{"playlistId": "p1", "title": "One"}, {"playlistId": "p2", "title": "Two"}]
+    )
+    service = YouTubeMusicService(_browser_file(tmp_path), lambda _auth: client)
+
+    assert service.library_playlists() == [Playlist("p1", "One"), Playlist("p2", "Two")]
+    assert client.limits == [None]
+
+
+def test_library_playlists_skips_entries_without_id_or_title(tmp_path: Path) -> None:
+    client = _FakeClient(
+        [{"title": "No id"}, {"playlistId": "p"}, {"playlistId": "ok", "title": "T"}]
+    )
+    service = YouTubeMusicService(_browser_file(tmp_path), lambda _auth: client)
+
+    assert service.library_playlists() == [Playlist("ok", "T")]
+
+
+def test_library_playlists_raises_when_browser_file_missing(tmp_path: Path) -> None:
+    service = YouTubeMusicService(tmp_path / "browser.json", lambda _auth: _FakeClient([]))
+
+    with pytest.raises(BrowserFileMissingError) as info:
+        service.library_playlists()
+
+    message = str(info.value)
+    for step in ("Firefox", "music.youtube.com", "browse", "ytmusicapi browser", "pbpaste"):
+        assert step in message
+
+
+def test_library_playlists_raises_authentication_error_when_factory_fails(tmp_path: Path) -> None:
+    def factory(_auth: str) -> YTMusicLike:
+        raise YTMusicUserError("bad headers")
+
+    service = YouTubeMusicService(_browser_file(tmp_path), factory)
+
+    with pytest.raises(AuthenticationError, match="ytmusicapi browser") as info:
+        service.library_playlists()
+    assert isinstance(info.value.__cause__, YTMusicUserError)
+
+
+@pytest.mark.parametrize("error", [YTMusicUserError("x"), KeyError("cookie"), ValueError("bad")])
+def test_library_playlists_raises_authentication_error_when_listing_fails(
+    tmp_path: Path, error: Exception
+) -> None:
+    client = _FakeClient([], error)
+    service = YouTubeMusicService(_browser_file(tmp_path), lambda _auth: client)
+
+    with pytest.raises(AuthenticationError):
+        service.library_playlists()
+
+
+def test_library_playlists_builds_the_client_once(tmp_path: Path) -> None:
+    calls: list[str] = []
+
+    def factory(auth: str) -> YTMusicLike:
+        calls.append(auth)
+        return _FakeClient([])
+
+    service = YouTubeMusicService(_browser_file(tmp_path), factory)
+    service.library_playlists()
+    service.library_playlists()
+
+    assert len(calls) == 1

tests/unit/presentation/test_cli.py

@@ -2,10 +2,17 @@
 @brief Tests of the command line with a fake source and injected input and output.
 """
 
-from conftest import FakeChartSource
+from pathlib import Path
+
+from conftest import FakeChartSource, FakeMusicService
 
 from musical_time_machine.constants import CHART_SIZE, DATE_PROMPT
-from musical_time_machine.domain.errors import ChartParseError, ChartUnavailableError
+from musical_time_machine.domain.errors import (
+    BrowserFileMissingError,
+    ChartParseError,
+    ChartUnavailableError,
+)
+from musical_time_machine.domain.playlist import Playlist
 from musical_time_machine.domain.song import Song
 from musical_time_machine.presentation.cli import main
 
@@ -57,3 +64,49 @@ def test_main_returns_one_when_chart_has_no_songs() -> None:
 
     assert code == 1
     assert lines == ["empty"]
+
+
+def test_main_check_login_prints_playlist_count() -> None:
+    lines: list[str] = []
+    service = FakeMusicService([Playlist("a", "A"), Playlist("b", "B"), Playlist("c", "C")])
+
+    code = main(["--check-login"], music_service=service, write=lines.append)
+
+    assert code == 0
+    assert lines == ["Found 3 playlists in your library."]
+
+
+def test_main_check_login_returns_one_when_browser_file_missing() -> None:
+    lines: list[str] = []
+    service = FakeMusicService(error=BrowserFileMissingError("browser.json was not found."))
+
+    code = main(["--check-login"], music_service=service, write=lines.append)
+
+    assert code == 1
+    assert lines == ["browser.json was not found."]
+
+
+def test_main_check_login_with_missing_real_file_returns_one(tmp_path: Path) -> None:
+    lines: list[str] = []
+    argv = ["--check-login", "--browser-file", str(tmp_path / "none.json")]
+
+    code = main(argv, write=lines.append)
+
+    assert code == 1
+    assert "ytmusicapi browser" in lines[0]
+
+
+def test_main_without_flag_does_not_use_the_music_service() -> None:
+    lines: list[str] = []
+    service = FakeMusicService(error=BrowserFileMissingError("must not be called"))
+
+    code = main(
+        [],
+        source=FakeChartSource([Song("T", "A")]),
+        music_service=service,
+        read=lambda _prompt: "2026-04-18",
+        write=lines.append,
+    )
+
+    assert code == 0
+    assert lines == ["1. T - A"]